The other day a mate in class asked for the origin of this word, he said that it could be related to people travelling from Britain to India, as it is.
The story goes that the more well-to-do passengers travelling to and from India used to have POSH written against their bookings, standing for 'Port Out, Starboard Home' (indicating the more desirable cabins, on the shady side of the ship).
Unfortunately, this story did not make its appearance until the 1930s, when the term had been in use for some twenty years, and the word does not appear to have been recorded in the form 'P.O.S.H.', which would be expected if it had originated as an abbreviation. Despite exhaustive enquiries by the late Mr George Chowdharay-Best, researcher for the OED, including interviews with former travellers and inspection of shipping company documents, no supporting evidence has been found.